First we consider a dielectric slab to elucidate the role of oblique incidence, i.e., angle between the two impinging linearly polarized plane waves, on the radiation pressure by using the analytically derived expression given in Eq. (REF ). For the sake of consistency, one can get optical force expression on a dielectric slab due to the normal incidence by setting \(\theta =0\) . In this scenario, no interference occurs REF (b), and the incident fields exert a positive acceleration on the center of mass of the slab by virtue of the law of conservation of momentum. Thus, it manifests that the radiation pressure by two incident normal plane waves is pushing for \({\rm Im}(n_s)\ge 0\)  [1]}, [2]}, [3]}.  Moreover, for better illustration, we calculate the normalized radiation pressure as a function of \(d/\lambda \)  for fixed real refractive index and the results are displayed in Fig. REF : (a) \({\rm Re}{(n_s)}=3.54\) ; and (b) \({\rm Re}{(n_s)}=1.5\) . Whereas, in both the cases different values of imaginary part of the refractive index have been used as: \( {\rm Im}(n_s)= 0.05 \)  (red line), \( {\rm Im}(n_s)= 0\)  (dashed line), and \( {\rm Im}(n_s)= -0.05 \)  (black line). It is clear that for passive slab, radiation pressure is positive for \({\rm Im}(n_s)\ge 0\) . On the other hand, pressure is negative when slab is active [4]}, [5]}, [6]} i. e. \({\rm Im}(n_s)<0\)  as demonstrated in Fig. REF  (black lines). Since there is no interference and the pressure is constant over the slab REF (b). Thus, the center of mass of the slab undergoes to a net pushing force when slab is passive and pulling force when slab is active. Another interesting feature is seen when we increase the thickness of the slab, that is, the force is periodic for a lossless slab because no photons are captured by the lossless slab, however, there is periodically increasing positive pressure for the absorptive case, due to absorptive losses. For active slab, the negative radiation pressure increases by increasing the \(d\)  due to the fact that a thicker active slab emits more light than it receives which allows for higher amplification of the forward momentum, consequently increasing the negative radiation pressure.
